Mexico uses SI, but understands Imperial vaguely for trade with the US or for uses like construction where parts were standardized in Imperial units. Last time I was there, I needed PVC pipe and had to ask for it in pulgadas (inches) of diameter and metros (meters) of length.
@ Shodan1980: Inches of Hg is a non-SI unit of measure equal to the height of a column of mercury at 1 g which would create equal pressure to the measured pressure. So a column of mercury 760 mm tall (760 torr) would be 29.9 inches of mercury (760 mm --> 29.9 inches) which would equal 1 atm or however many bar or Pa (~100k), where the Pa is an SI unit (1 N/m^2). And if you think that's weird, in medicine there's measures of pressure in both mm of H20 (intrathoracic) and mm of Hg (blood pressure, oncotic pressures, gas pressures inside blood and the lungs, etc).
Why? Not a freaking clue.
A little off topic, but some more of medicine's weirdness, at least in the US.
Some medicines come in international units, or IUs. IUs measures pharmacological properties and has to be defined individually for each drug or vitamin by the WHO. Some dosing is an artifact from when things were doled out in grains, such as 325 mg of aspirin (5 grains). We get milli-, micro, and nanograms/dL, a wonky measure of concentration vs. the SI's molarity and molality. We get scales which sound objective but really are just clinical impressions (a grade II/VI holosystolic murmur and 2+ pitting edema - sounds objective, right?), or measures of things that don't exist (the drug has a volume of distribution of 200 L, in a 70 kg patient). Needles are in non-SI units with inches for length and gauge for width, though of course the contents are usually in SI. Catheters and endotracheal tubes are measured in French, another wonky scale. Body Mass Index is based of SI units, but almost always reported without units and can be taken from Imperial measures. In urine, the presence of white blood cells is measured in cells per microscopic field in many cases. Concentrations of IV fluids are listed in percentages and SI, but everyone refers to it in terms of an artificial unit close to normal human osmolarity. Normal saline is NaCl in water at a concentration -close- to the normal human, and you frequently refer to "half-saline," "quarter saline," "normal saline," or other fluids as compared to that value.
And everyone knows about the Bristol Stool Scale [http://www.sthk.nhs.uk/library/documents/stoolchart.pdf].